The present invention relates generally to stents for implanting into a living body. In particular, the present invention relates to a biological covering for a stent suitable for implanting into a variety of lumens.
In an attempt to prevent restenosis, metallic vascular stents have been permanently implanted in coronary or peripheral vasculature. These stents are typically delivered intraluminally by a catheter and expanded in place to support a diseased portion of an artery.
One shortcoming of these conventional stents is that even after stent implantation, restenosis can still occur. Another shortcoming is that during the implantation of the stent, the stent may cause particles to discharge from the artery wall through the open cell. These dislodged particles can embolize in the bloodstream, and may cause catastrophic effects.
In an attempt to reduce these problems, coverings have been proposed for stents. These coverings have been made from artificial materials, such as PTFE. As of yet, however, coverings made from artificial materials have not proven successful. This may be because of the poor biocompatibility of such materials.
There is also some experience using biological tissue such as bovine pericardium to build a covering for a stent that is more biocompatible than coverings made from artificial materials. Preliminary results with bovine pericardium have been encouraging from the point of view of biocompatibility.
The current method, however, of creating a covering using biological tissue is simplistic. A rectangular piece of pericardium is harvested from a bovine source. The pericardium is then, after being prepared so that it is suitable for implantation, rolled into a cylinder. The abutting edges of the pericardium are sewn together to create a covering. This covering is then placed over a stent.
There is an inherent disadvantage in this cut and sew approach to creating a covering. Biological tissue has a very small expansion range. Therefore, the diameter of the cut and sewn cylinder of tissue is limited to a very small range. This limits the stent to a very small range of expansion diameters, limiting the utility of the stent. This also limits the difference in diameter between the stent as delivered and the stent at its expanded state, increasing significantly the profile of the delivery system required for a given supported diameter.
In one embodiment of the present invention, biological fibers or biodegradable fibers or fiber groups are arranged as interwoven threads to make an expandable tube. The interwoven threads are arranged with an acute angle between the interwoven threads while the stent is unexpanded. When the stent is expanded, the angle between the interwoven threads increases. This allows the stent covering to be expanded to a variety of diameters.
In another embodiment, a strip of pericardium is helically wound around a stent with a substantial overlap between adjacent windings while the stent is in a first, unexpanded diameter. During expansion to a second expanded diameter, the strips will slide over the stent and unwrap for a smaller number of loops, but will still completely cover the stents. In a further enhancement to this embodiment, the edges of the spiral wrapping are formed into locking folds to prevent the spiral loops from separating during expansion of the stent and covering.
Accordingly, it is an object of the current invention to provide an improved biological tissue covering for a stent, and a method for producing the same.
Further objectives and advantages of the subject invention will be apparent to those skilled in the art from the detailed description of the disclosed invention.